Binder compositions based upon aqueous alkali metal silicate solutions have been known for a long time and are used on a large scale in manufacturing. The alkali metal silicate solutions, particularly sodium silicate solutions; usually have a molar ratio of SiO.sub.2 :Me.sub.2 O (Me=alkali metal) in the range from about 2.0:1 to 3.4:1 as well as a solids content of from about 35 to 50 percent by weight. Binder compositions of this type are used, for example, in the preparation of mineral insulating materials, impregnating substances, coating materials, paints, and putties as well as in glues for the gluing of wood, paper, ceramics, and mineral materials.
Such binders can also be used for the preparation of molds for metal casting. For this purpose, the aqueous alkali metal silicate solution usually is mixed with fine fillers such as sand, and the resulting mixture is molded into the desired shape and hardened with the aid of hardeners. Suitable hardeners for this purpose include, for example, carbon dioxide gas, compounds with acid reaction, or acid-evolving compounds. A considerable disadvantage of this method is that it is extremely difficult to remove the finished forms, that is, the molds and cores, from the solidified castings after metal casting.
Various carbohydrates, such as sugar, molasses, starch, or starch or cellulose derivatives, usually are added to the binders to facilitate breaking of the molds after the completed casting. However, these additives, which may be contained in amounts of up to about 10 percent by weight in the binder composition, generally result in an undesirable absorption of water by the hardened molds during storage. This leads to a considerably reduced durability of the molds, particularly around their edges.
Consequently, there has been a need to develop a binder based upon aqueous alkali metal silicate solutions that is particularly suitable for the preparation of molds for metal casting without the disadvantages listed above.